cmd/go: fix detection of ELF binaries

Change-Id: I0e6f1a39b3d6b15d05891c8b25ab6644356bde5f
Reviewed-on: https://go-review.googlesource.com/10751Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/pkg.go

@@ -1166,7 +1166,7 @@ var (
 	goBuildPrefix = []byte("\xff Go build ID: \"")
 	goBuildEnd    = []byte("\"\n \xff")
 
-	elfPrefix = []byte("ELF\x7F")
+	elfPrefix = []byte("\x7fELF")
 )
 
 // readBuildIDFromBinary reads the build ID from a binary.